410 PRACTICAL PHYSIOLOGY 



taken by the mouth, and when the diet is an exclusively vegetable one. 

 Lastly, an alkaline reaction may be due to ammonia, which is produced 

 by microbal hydrolysis of urea (see Urea, p. 417). For this reason 

 stale urine always reacts alkaline. If the alkaline reaction of freshly 

 passed urine is due to ammonia, decomposition must be taking place in 

 the bladder. 



Colour. The straw colour of healthy urine is due to Urochrome, the 

 origin of which is uncertain. Another pigment, Urobilin, is normally 

 present in traces in the urine. In fever, and when there is liver disease 

 or rapid destruction of haemoglobin, the amount of urobilin is often 

 much increased, imparting a reddish tint to the urine. Urobilin is 

 identical with stercobilin, the pigment of faeces, which is formed from 

 bilirubin in the intestine by bacterial action, so that its presence in 

 urine is presumably due to absorption from the intestine. Urobilin 

 can also exist in the urine as a colourless precursor, or chromogen, which 

 yields the pigment, when the urine is acidified with sulphuric acid and 

 allowed to stand. 



EXPERIMENT III. Examine an acid solution of urobilin or sterco- 

 bilin with the spectroscope. A diffuse absorption band is seen between 

 the green and the violet of the spectrum. 



A third pigment is uroerythrin, the colouring matter of pink urate 

 deposits. It appears to be related to skatoxyl sulphuric acid. 

 Haematoporphyrin (see Haemoglobin, p. 350) may also occur. 



Indican, or indoxylsulphuric acid (see Ethereal Sulphates, p. 437), is 

 the cause of the blue tint sometimes acquired by urine on standing, as 

 it is oxidised to indigo blue. Normally a small quantity is present. 

 This is increased when there is excessive putrefaction in the small 

 intestine, or in an abscess. 



EXPERIMENT IV. Test urine for indican. Mix about 5 c.c. of 

 urine in a test tube with an equal volume of strong hydrochloric acid 

 containing 0'4 per cent, ferric chloride. Add about 3 c.c. of chloroform 

 and shake the tube. The chloroform on settling will be tinged with 

 blue (indigo) if the urine contains indican. 



The Nitrogenous Constituents. Over 90 per cent, of the nitrogen 

 in combination excreted by the body is present in the urine, the 

 remainder occurring in the faeces (about 1 gramme per diem), and as 

 urea in the sweat. A determination of the total nitrogen of the urine is, 

 therefore, of great importance. The method employed is that of 

 Kjeldahl 



EXPERIMENT V. Measure 5 c.c. of urine with a pipette into a Jena 

 flask. (This flask should be of at least 400 c.c. capacity. It saves 

 time and chance of error to use the flask of 750-1000 c.c. capacity, 



